Carbon dioxide reduction device and method for reducing carbon dioxide

ABSTRACT

A device for reducing CO 2  by light, including: a cathode chamber holding a first electrolyte solution that contains CO 2 ; an anode chamber holding a second electrolyte solution; a proton conducting membrane disposed in a connecting portion between these chambers; a cathode electrode; and an anode electrode. The cathode electrode has a CO 2  reduction reaction region composed of a metal or a metal compound, and the anode electrode has a photochemical reaction region composed of nitride semiconductors. The photochemical reaction region of the anode electrode has a multilayer structure of a GaN layer and an Al x Ga 1-x N layer containing Mg (0&lt;x≦0.25). The content of Mg in the Al x Ga 1-x N layer is 1×10 15  or more and 1×10 19  or less in terms of the number of Mg atom per cm 3 . The anode electrode is disposed in such a manner that the Al x Ga 1-x N layer can be exposed to light.

This is a continuation of International Application No. PCT/JP2014/002526, with an international filing date of May 13, 2014, which claims the foreign priority of Japanese Patent Application No. 2013-100944, filed on May 13, 2013, the entire contents of both of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to a carbon dioxide reduction device for reducing carbon dioxide by light energy and to a method for reducing carbon dioxide using this device.

2. Description of Related Art

Carbon dioxide (CO₂) is a substance that plays an important role in reserving carbon atoms in the global carbon cycle. Given that CO₂ is a reservoir of carbon atoms, it is also a substance that can serve as a carbon source for various carbon compounds as typified by organic compounds. However, since CO₂ is an energetically very stable substance, the use of CO₂ as a carbon source requires a high level of reduction energy.

Meanwhile, there are growing concerns about the increase in the CO₂ concentration in the atmosphere due to the consumption of fossil fuels such as coal, petroleum, and natural gas, and about the global climate change (so-called global warming) due to the increase in the CO₂ concentration. Reduction of CO₂ by light energy has attracted attention not only from the viewpoint of using CO₂ as a carbon source but also from the viewpoint of reducing the consumption of fossil fuels by the use of carbon compounds converted from CO₂ so as to suppress the climate change.

Various methods have been tried to reduce CO₂ by light energy. The following documents disclose methods for reducing CO₂ by light energy.

JP 55(1980)-105625 A and JP 2526396 B2 each disclose a method of using an oxide semiconductor such as titania or zirconia as a catalyst for CO₂ reduction, more specifically, a method of irradiating a suspension obtained by dispersing the oxide semiconductor powder in water with light while introducing CO₂ into the suspension.

JP 3876305 B2 and JP 4158850 B2 each disclose a method of using a composite compound of a metal component and a semiconductor component such as a titanium compound, as a catalyst for CO₂ reduction, more specifically, a method of introducing CO₂ into a suspension obtained by dispersing powder of the composite compound in water and then irradiating the suspension with light.

JP 2010-064066 A discloses a method of using, as a catalyst for CO₂ reduction, a catalyst in which a semiconductor and a base material such as an organic rhenium complex or an organic ruthenium complex are joined so that they can donate and accept electrons to and from each other, more specifically, a method of introducing CO₂ into a suspension obtained by dispersing powder of the catalyst in an organic solvent and then irradiating the suspension with light.

JP 2011-094194 A discloses a photochemical reaction device including an oxidation reaction electrode for oxidizing water to produce oxygen and a reduction reaction electrode electrically connected to the oxidation reaction electrode and for reducing carbon dioxide to synthesize a carbon compound. JP 2011-094194 A also discloses titania, tungsten oxide, and tantalum oxynitride as the materials for the oxidation reaction electrode, and the catalyst disclosed in JP 2010-064066 A as the material for the reduction reaction electrode. In the device disclosed in JP 2011-094194 A, both electrodes are irradiated with light.

JP 05(1993)-311476 A and JP 07(1995)-188961 A each disclose an electrochemical reduction device including an anode electrode made of an oxide semiconductor such as titania and a cathode electrode having a specific structure made of a specific metal, and a method for reducing CO₂ on the cathode electrode by irradiating the anode electrode with light in this device. The device disclosed in JP 05(1993)-311476 A or JP 07(1995)-188961 A requires an external power source such a solar cell or a potentiostat disposed between the anode electrode and the cathode electrode.

WO 2012/046374 A1 discloses a method for reducing CO₂ on a cathode electrode by irradiating an anode electrode with light in a device including the anode electrode whose surface has an area of a nitride semiconductor such as gallium nitride or aluminum gallium nitride and the cathode electrode made of a metal or a metal compound. The method disclosed in WO 2012/046374 A1 does not require an external power source between the anode electrode and the cathode electrode.

WO 2006/082801 A1 discloses a nitride-based semiconductor photocatalyst represented by the formula Al_(y)Ga_(1-x-y)In_(x)N (x-y≦0.45, 0≦x≦1, and 0≦y≦1) for use as an electrode for a device for producing acidic water and alkaline water, although it does not disclose a method for reducing CO₂ by light energy.

SUMMARY OF THE INVENTION

The widespread use of devices and methods for reducing CO₂ by light energy to convert CO₂ into carbon compounds, like plant photosynthesis, would be very useful for industrial development and global environmental protection. However, in conventional devices and methods, the efficiency of reducing CO₂ by light energy to convert it into carbon compounds without the use of any external power source is not necessarily high enough.

One non-limiting and exemplary embodiment of the present disclosure provides a CO₂ reduction device for reducing CO₂ by light energy and a method for reducing CO₂ by light energy, in which CO₂ can be reduced without the use of any external power source and CO₂ can be converted into carbon compounds with higher efficiency than conventional devices and methods.

Additional benefits and advantages of the disclosed embodiments will be apparent from the specification and Figures. The benefits and/or advantages may be individually provided by the various embodiments and features of the specification and drawings disclosure, and need not all be provided in order to obtain one or more of the same.

In one general aspect, the techniques disclosed here feature a CO₂ reduction device for reducing CO₂ by light energy. This device includes: a cathode chamber holding a first electrolyte solution that contains CO₂; an anode chamber holding a second electrolyte solution, connected to the cathode chamber; a proton conducting membrane that is disposed in a connecting portion between the anode chamber and the cathode chamber so as to serve as a separator between the first electrolyte solution and the second electrolyte solution and to conduct hydrogen ions between the first and second electrolyte solutions; a cathode electrode disposed in the cathode chamber so as to be in contact with the first electrolyte solution; and an anode electrode disposed in the anode chamber so as to be in contact with the second electrolyte solution. The cathode electrode has a CO₂ reduction reaction region that is in contact with the first electrolyte solution and is composed of a metal or a metal compound. The anode electrode has a photochemical reaction region that is in contact with the second electrolyte solution and is composed of nitride semiconductors. The photochemical reaction region of the anode electrode has a multilayer structure of a GaN layer and an Al_(x)Ga_(1-x)N layer containing Mg (0<x≦0.25). The content of Mg in the Al_(x)Ga_(1-x)N layer is 1×10¹⁵ or more and 1×10¹⁹ or less in terms of the number of Mg atoms that are contained in a unit volume (1 cm³) of the Al_(x)Ga_(1-x)N layer. The anode electrode is disposed in the anode chamber in such a manner that the Al_(x)Ga_(1-x)N layer in the photochemical reaction region can be exposed to light. The cathode electrode and the anode electrode are electrically connected to each other without an external power source interposed therebetween.

These general and specific aspects may be implemented using a system, a method, and a computer program, and any combination of systems, methods, and computer programs.

The CO₂ reduction device and the method for reducing CO₂ of the present disclosure are the device and method for reducing CO₂ by light energy, in which CO₂ can be reduced without the use of any external power source and CO₂ can be converted into carbon compounds with higher efficiency than conventional devices and methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view schematically showing an example of an anode electrode included in a CO₂ reduction device of the present disclosure.

FIG. 1B is a cross-sectional view schematically showing another example of the anode electrode included in a CO₂ reduction device of the present disclosure.

FIG. 1C is a cross-sectional view schematically showing still another example of the anode electrode included in a CO₂ reduction device of the present disclosure.

FIG. 1D is a cross-sectional view schematically showing even still another example of the anode electrode included in a CO₂ reduction device of the present disclosure.

FIG. 2A is a cross-sectional view schematically showing an example different from the example given above, of the anode electrode included in a CO₂ reduction device of the present disclosure.

FIG. 2B is a cross-sectional view schematically showing an example different from the example given above, of the anode electrode included in a CO₂ reduction device of the present disclosure.

FIG. 2C is a cross-sectional view schematically showing an example different from the example given above, of the anode electrode included in a CO₂ reduction device of the present disclosure.

FIG. 2D is a cross-sectional view schematically showing an example different from the example given above, of the anode electrode included in a CO₂ reduction device of the present disclosure.

FIG. 3 is a diagram schematically showing an example of the CO₂ reduction device of the present disclosure and an example of the CO₂ reduction method of the present disclosure using this device.

FIG. 4 is a diagram showing the amounts of CO₂ reduced per unit time evaluated in Examples 1 to 3 and Comparative Example 1.

FIG. 5 is a diagram showing the relationship between the content of Mg atoms in an AlGaN layer of the anode electrode and the amount of CO₂ reduced per unit time, evaluated in Example 7.

DETAILED DESCRIPTION

A first aspect of the present disclosure provides a CO₂ reduction device for reducing CO₂ by light energy, including: a cathode chamber holding a first electrolyte solution that contains CO₂; an anode chamber holding a second electrolyte solution, connected to the cathode chamber; a proton conducting membrane that is disposed in a connecting portion between the anode chamber and the cathode chamber so as to serve as a separator between the first electrolyte solution and the second electrolyte solution and to conduct hydrogen ions between the first and second electrolyte solutions; a cathode electrode disposed in the cathode chamber so as to be in contact with the first electrolyte solution; and an anode electrode disposed in the anode chamber so as to be in contact with the second electrolyte solution, wherein the cathode electrode has a CO₂ reduction reaction region that is in contact with the first electrolyte solution and is composed of a metal or a metal compound, the anode electrode has a photochemical reaction region that is in contact with the second electrolyte solution and is composed of nitride semiconductors, the photochemical reaction region of the anode electrode has a multilayer structure of a GaN layer and an Al_(x)Ga_(1-x)N layer containing Mg (0<x≦0.25), a content of Mg in the Al_(x)Ga_(1-x)N layer is 1×10¹⁵ or more and 1×10¹⁹ or less in terms of the number of Mg atoms that are contained in a unit volume (1 cm³) of the Al_(x)Ga_(1-x)N layer, the anode electrode is disposed in the anode chamber in such a manner that the Al_(x)Ga_(1-x)N layer in the photochemical reaction region can be exposed to light, and the cathode electrode and the anode electrode are electrically connected to each other without an external power source interposed therebetween.

A second aspect of the present disclosure provides the CO₂ reduction device according to the first aspect, wherein the content of Mg in the Al_(x)Ga_(1-x)N layer is 1×10¹⁶ or more and 1×10¹⁸ or less in terms of the number of Mg atoms that are contained in the unit volume (1 cm³) of the Al_(x)Ga_(1-x)N layer.

A third aspect of the present disclosure provides the CO₂ reduction device according to the first or second aspect, wherein the x has a value of 0.10 or more and 0.15 or less.

A fourth aspect of the present disclosure provides the CO₂ reduction device according to any one of the first to third aspects, wherein the GaN layer is composed of an n-type GaN.

A fifth aspect of the present disclosure provides the CO₂ reduction device according to any one of the first to fourth aspects, wherein a metal oxide containing Ni is disposed on the Al_(x)Ga_(1-x)N layer in the photochemical reaction region.

A sixth aspect of the present disclosure provides the CO₂ reduction device according to the fifth aspect, wherein the metal oxide is in the form of fine particles.

A seventh aspect of the present disclosure provides the CO₂ reduction device according to any one of the first to sixth aspects, wherein the metal constituting the reduction reaction region includes at least one selected from copper, gold, silver, tantalum, and indium.

An eighth aspect of the present disclosure provides the CO₂ reduction device according to any one of the first to seventh aspects, wherein the first electrolyte solution is an aqueous solution containing at least one electrolyte selected from potassium bicarbonate, sodium bicarbonate, potassium chloride, and sodium chloride.

A ninth aspect of the present disclosure provides a method for reducing CO₂ using a CO₂ reduction device, wherein the device is the CO₂ reduction device according to any one of the first to eighth aspects, and the method includes the step of irradiating the Al_(x)Ga_(1-x)N layer in the photochemical reaction region of the anode electrode with light having a wavelength of 365 nm or less, with the first electrolyte solution and the second electrolyte solution being held in the cathode chamber and the anode chamber respectively, so as to allow generation of electrons and hydrogen ions to proceed in the photochemical reaction region and to allow a reaction of reducing CO₂ contained in the first electrolyte solution to proceed in the reduction reaction region of the cathode electrode.

A tenth aspect of the present disclosure provides the method for reducing CO₂ according to the ninth aspect, further including the step of introducing a gas containing carbon dioxide into the first electrolyte solution held in the cathode chamber.

An eleventh aspect of the present disclosure provides the method for reducing CO₂ according to the ninth or tenth aspect, wherein the step is performed with the device being placed at room temperature and atmospheric pressure.

A twelfth aspect of the present disclosure provides the method for reducing CO₂ according to any one of the ninth to eleventh aspects, wherein the reaction of reducing carbon dioxide produces at least one selected from methanol, ethanol, acetaldehyde, formic acid, methane, ethylene, and carbon monoxide.

Conventionally, methods for reducing CO₂ by light energy are known. In a method (see JP 55(1980)-105625 A, JP 2526396 B2, JP 3876305 B2, JP 4158850 B2, or JP 2010-064066 A) in which a suspension is prepared by dispersing semiconductor powder in a solution containing CO₂ so as to allow the powder to act as a catalyst for CO₂ reduction, since carriers (electrons and holes) generated in the catalyst by light irradiation are easily recombined before they are used to reduce CO₂, highly efficient CO₂ reduction cannot be achieved. On the other hand, in a method (see JP 2011-094194 A, JP 05(1993)-311476 A, or JP 07(1995)-188961 A) in which an oxidation reaction electrode (anode electrode) for oxidizing water to evolve oxygen and a reduction reaction electrode (cathode electrode) electrically connected to the anode electrode and for reducing carbon dioxide to synthesize a carbon compound, since electrons and holes generated at the electrode by light irradiation are immediately separated and their recombination is suppressed, more efficient CO₂ reduction is expected.

Here, in the CO₂ reduction reaction using light energy, the amount of reduction products obtained by the CO₂ reduction depends on the level of photoelectromotive force generated in an anode electrode as an electrode to be irradiated with light (i.e., a photochemical electrode) and on the amount of carriers generated by photoexcitation of this electrode. The above-described suppression of the recombination of carriers increases the amount of carriers generated. However, when the anode electrode is made of an oxide semiconductor such as titania, the energy level of electrons photoexcited at the electrode does not sufficiently reach an energy level necessary for reducing CO₂. Therefore, it is necessary to dispose an external power source such as a solar cell or a potentiostat between the anode electrode and the cathode electrode.

In contrast, in the device and the method of the present disclosure, the use of a nitride semiconductor in the anode electrode increases the energy level of excited electrons, and thus allows the CO₂ reduction reaction to proceed without the help of an external power source to increase the potential. In addition, in the device and the method of the present disclosure, a multilayer structure of a GaN layer and an Al_(x)Ga_(1-x)N layer (0<x≦0.25) containing a specific amount of Mg is adopted. Thereby, the built-in potential formed at the interface between these layers, that is, the magnitude of the internal electric field, increases. This increase further suppresses the recombination of carriers generated by photoexcitation, and thus increases the level of the photoelectromotive force in the anode electrode and the amount of carriers generated. This means that the device and the method of the present disclosure each have a mechanism for reducing the loss of carriers excited in the anode electrode as a photochemical electrode and for increasing the level of the photoelectromotive force in this electrode. Such a mechanism has not been disclosed so far. The effect of this mechanism is observed, for example, as an increase in the value of a current flowing from the anode electrode to the cathode electrode upon irradiation with light (i.e., an increase in the amount of carriers supplied from the anode electrode to the cathode electrode). This effect contributes to the achievement of more efficient CO₂ reduction in the device and the method of the present disclosure.

WO 2006/082801 A1 discloses an electrode made of Mg-containing Al_(y)Ga_(1-x-y)In_(x)N (x-y≦0.45, 0≦x≦1, and 0≦y≦1). However, this electrode is a cathode electrode, and in WO 2006/082801 A1, water reduction reaction is allowed to proceed by irradiating the cathode electrode with light. In contrast, in the device and the method of the present disclosure, a Mg-containing Al_(y)Ga_(1-x)N layer is used in the anode electrode and water oxidation reaction is allowed to proceed by irradiating the anode electrode with light. Therefore, the device and the method according to the present disclosure are based on a technical idea completely different from that of the disclosure of WO 2006/082801 A1.

[CO₂ Reduction Device]

(Anode Electrode) FIG. 1A to FIG. 1D each show an example of an anode electrode used in the device and the method of the present disclosure. In the anode electrode, carriers (electrons and holes) are generated by light irradiation. The generated electrons move to the cathode electrode electrically connected to the anode electrode. The generated holes are used for water oxidation reaction at the anode electrode, and hydrogen ions (protons) generated in the reaction move diffusively toward the cathode electrode through an anode-side electrolyte solution (second electrolyte solution), a proton conducting membrane disposed in a connecting portion between an anode chamber and a cathode chamber, and a cathode-side electrolyte solution (first electrolyte solution). At the cathode electrode, CO₂ reacts with electrons and protons so as to allow the CO₂ reduction reaction at this electrode to proceed. Focusing on this generation of carriers by light irradiation, this anode electrode is a photochemical electrode for CO₂ reduction. Focusing on the formation of oxygen by water oxidation reaction, this anode electrode is an oxygen generating electrode.

An anode electrode 10 a shown in FIG. 1A is a multilayer body of a Mg-containing Al_(x)Ga_(1-x)N layer 11, a GaN layer 12, an electrically conductive substrate 13, and an electrode layer 14.

The Al_(x)Ga_(1-x)N layer 11 is the layer where carriers (electrons and holes) are generated by light irradiation. In other words, upon absorption of light in the Al_(x)Ga_(1-x)N layer 11, photoexcitation occurs therein and thereby carriers are generated. The generated carriers contribute to oxidation-reduction reaction as described above. The holes generated in the Al_(x)Ga_(1-x)N layer 11 move to the surface of the anode electrode 10 a, typically to the surface of the Al_(x)Ga_(1-x)N layer 11, and oxidizes water in contact with the anode electrode 10 a to produce protons and oxygen. The protons thus produced move diffusively in the second electrolyte solution with which the anode electrode 10 a is in contact, and oxygen as a gas leaves the anode electrode 10 a.

The value of the band gap, that is, the width of the band gap of the Al_(x)Ga_(1-x)N layer 11 is 3.4 eV or more. Therefore, the Al_(x)Ga_(1-x)N layer 11 of the anode electrode 10 a needs to be irradiated with light having a wavelength of 365 nm or less whose energy is equal to or higher than that corresponding to this band gap.

The content (doping concentration) of Mg in the Al_(x)Ga_(1-x)N layer 11 is 1×10¹⁵ or more and 1×10¹⁹ or less in terms of the number of Mg atoms that are contained in a unit volume (1 cm³) of the Al_(x)Ga_(1-x)N layer 11 (hereinafter also referred to as “the number of atoms per cm³”).

Desirably, the content of Mg in the Al_(x)Ga_(1-x)N layer 11 is 1×10¹⁶ or more and 1×10¹⁸ or less in terms of the number of Mg atoms that are contained in the unit volume (1 cm³) of the Al_(x)Ga_(1-x)N layer 11. In this case, the effect obtained by adding Mg is further enhanced. Specifically, the photoelectromotive force and the carrier utilization efficiency in the Al_(x)Ga_(1-x)N layer 11 are enhanced, and thereby the efficiency of the CO₂ reduction in the device and the method of the present disclosure is further enhanced.

When the content of Mg in the Al_(x)Ga_(1-x)N layer is less than 1×10¹⁵ in terms of the number of atoms per cm³, the effect of adding Mg cannot be obtained. On the other hand, when the content of Mg in the Al_(x)Ga_(1-x)N layer is more than 1×10¹⁹ in terms of the number of atoms per cm³, the characteristics of the Al_(x)Ga_(1-x)N layer change and the photoelectromotive force and the carrier utilization efficiency rather decrease in this layer (that is, in the anode electrode 10 a).

The Al_(x)Ga_(1-x)N constituting the Al_(x)Ga_(1-x)N layer 11 has a composition satisfying the formula 0<x≦0.25. The value of x in this range is suitable when a readily available light source (such as the sun and a xenon lamp) is used for irradiation of light having a wavelength of 365 nm or less. The value of x is desirably 0.10 or more and 0.15 or less. This desirable range of x values is particularly suitable when a common xenon lamp is used for the above-mentioned light irradiation. It should be understood that a xenon lamp may also be used as a light source when the x value is outside this desirable range.

The depth in the Al_(x)Ga_(1-x)N layer 11 that the light having a wavelength of 365 nm or less can reach (i.e., the distance from the irradiated surface of this layer 11) is approximately 100 nm, although it depends on the band gap value of Al_(x)Ga_(1-x)N. The depth (i.e., the thickness of the light absorption region of the Al_(x)Ga_(1-x)N layer 11) is parallel to the irradiated surface of the layer 11. In view of this, the thickness of the Al_(x)Ga_(1-x)N layer 11 is desirably 70 nm or more and 1000 nm or less, and more desirably 80 nm or more and 200 nm or less.

The GaN layer 12 is the layer for enhancing the carrier utilization efficiency in the Al_(x)Ga_(1-x)N layer 11 (that is, in the anode electrode 10 a) based on the multilayer structure of the GaN layer 12 and the Al_(x)Ga_(1-x)N layer 11. Presumably, an increase in the built-in potential formed at the interface between the Al_(x)Ga_(1-x)N layer 11 and the GaN layer 12 contributes to this enhancement.

Addition of an impurity (dopant) can reduce the electrical resistance of the GaN layer 12. This GaN layer 12 can serve as an electron conducting layer for efficiently transporting electrons that are one type of the carriers generated in the Al_(x)Ga_(1-x)N layer 11 by light irradiation. In this case, in the above-described multilayer structure of the anode electrode 10 a, the light absorbing layer (Al_(x)Ga_(1-x)N layer 11) and the electron conducting layer (GaN layer 12) are functionally separated. This separation promotes the extraction and transport of the carriers (electrons) generated in the light absorbing layer, and thus an even higher level of photoelectromotive force and an even higher carrier utilization efficiency are achieved.

In view of the function as an electron conducting layer, the GaN layer 12 is desirably a layer having a lower electrical resistance than the Al_(x)Ga_(1-x)N layer 11. This GaN layer 12 is a layer converted to n-type by addition of an impurity, that is, a layer composed of n-type GaN. The impurity (dopant) is silicon (Si), for example. The n-type includes n+-type.

The anode electrode 10 a has a photochemical reaction region that is composed of nitride semiconductors and that comes into contact with the second electrolyte solution when it is incorporated into a CO₂ reduction device. When the Al_(x)Ga_(1-x)N layer 11 in this region is irradiated with light, generation of carriers proceeds. The structure of the anode electrode 10 a is not limited as long as the photochemical reaction region has a multilayer structure of the Al_(x)Ga_(1-x)N layer 11 and the GaN layer 12. In the anode electrode 10 a shown in FIG. 1A, the above-described multilayer structure is formed over the entire electrode 10 a, and one of the principal surfaces of the electrode 10 a can serve entirely as a photochemical reaction region. The above-described multilayer structure may be formed in a part of the anode electrode 10 a, which means that a part of the principal surface of the anode electrode 10 a may be used as a photochemical reaction region.

In the anode electrode 10 a shown in FIG. 1A, the above-described multilayer structure is formed on one of the principal surfaces of the substrate 13. In an anode electrode for use in the CO₂ reduction device of the present disclosure, the above-described multilayer structure may be formed on both principal surfaces of the substrate 13.

The electrically conductive substrate 13 serves as a layer not only for increasing the strength, shape retention, and ease of handling of the anode electrode 10 a but also efficiently transporting electrons of the carriers generated in the Al_(x)Ga_(1-x)N layer 11.

The electrically conductive substrate 13 is composed of, for example, single-crystal gallium nitride (GaN), gallium oxide (Ga₂O₃), single-crystal silicon (Si), silicon carbide (SiC), zinc oxide (ZnO), or zirconium boride (ZrB₂).

The anode electrode for use in the device and the method of the present disclosure does not necessarily need a substrate like the electrically conductive substrate 13. The device can use an anode electrode including the electrically conductive substrate 13, as needed.

The electrode layer 14 is a layer composed of an electrically conductive material, and serves as a terminal for extracting, from the anode electrode 10 a, electrons of the carriers generated in the Al_(x)Ga_(1-x)N layer 11. The structure of the electrode layer 14 is not limited as long as it serves as the terminal. As for the position of the electrode layer 14, in the example shown in FIG. 1A, the electrode layer 14 is formed on the entire surface of the electrically conductive substrate 13 opposite to the surface thereof facing the GaN layer 12. However, the electrode layer 14 may be formed on a part of this opposite surface. The electrode layer 14 may be formed on the surface of the electrically conductive substrate 13 facing the GaN layer 12 in such a manner that the electrode layer 14 and the GaN layer 12 do not overlap each other, or the electrode layer 14 may be formed on the GaN layer 12 in such a manner that the electrode layer 14 and the Al_(x)Ga_(1-x)N layer 11 do not overlap each other (for the latter case, see FIG. 2A).

The electrode layer 14 is composed of, for example, a metal or a metal compound. The metal is, for example, gold (Au), silver (Ag), copper (Cu), aluminum (Al), titanium (Ti), or an alloy of these metals.

The electrode layer 14 may be composed of a single electrically conductive film or may be a multilayer body composed of a plurality of electrically conductive films. The material constituting the electrically conductive film is, for example, the above-mentioned metal or metal compound.

When the electrically conductive substrate 13 is composed of a semiconductor or when the electrode layer 14 is formed on the semiconductor GaN layer 12, it is desirable that the electrode layer 14 or the surface (or the film) of the electrode layer 14 that is in contact with the electrically conductive substrate 13 or the GaN layer 12 be composed of a material with a low interfacial resistance to these semiconductors. This material is Ti, for example.

The anode electrode for use in the device and the method of the present disclosure does not necessarily need the electrode layer 14. The device can use an anode electrode including the electrode layer 14, as needed.

In the device and the method of the present disclosure, the Al_(x)Ga_(1-x)N layer 11 in the photochemical reaction region is irradiated with light. Therefore, when the Al_(x)Ga_(1-x)N layer 11 serves as the irradiated surface of the anode electrode 10 a, the efficiency of the carrier generation is high. When the Al_(x)Ga_(1-x)N layer 11 is used as the irradiated surface of the anode electrode 10 a, the Al_(x)Ga_(1-x)N layer 11, the GaN layer 12, the electrically conductive substrate 13, and the electrode layer 14 are stacked in this order from the irradiated surface of the anode electrode 10 a.

In the anode electrode for use in the device and the method of the present disclosure, a metal oxide may be disposed on the Al_(x)Ga_(1-x)N layer 11 in the photochemical reaction region. This metal oxide serves as a co-catalyst for increasing the efficiency of oxygen generation in the photochemical reaction region of the anode electrode. This metal oxide also serves as a protective layer of the Al_(x)Ga_(1-x)N layer 11. The metal oxide contains, for example, nickel (Ni) and desirably contains Ni as a main component. The main component refers to a component whose content is the highest. The content is usually 50 wt. % or more, desirably 60 wt. % or more, and more desirably 70 wt. % or more. The metal oxide may also be nickel oxide (NiO_(x), typically NiO or Ni₂O₃).

FIG. 1B to FIG. 1D each show an example of the anode electrode in which a metal oxide is disposed on the Al_(x)Ga_(1-x)N layer 11 in the photochemical reaction region.

An anode electrode 10 b shown in FIG. 1B has the same structure as the anode electrode 10 a of FIG. 1A, except that a surface coating layer 15 composed of the above-mentioned metal oxide is disposed on the Al_(x)Ga_(1-x)N layer 11 so as to cover the layer 11. This surface coating layer 15 is a layer having the property of transmitting at least a part of light within a bandwidth of wavelengths of 365 nm or less. In order to ensure this light transmitting property, the thickness of the surface coating layer 15 is desirably 10 nm or less. The surface coating layer 15 may contain fine particles of a metal or a metal oxide. In this case, the function as a co-catalyst and the function as a protective layer of the surface coating layer 15 are enhanced. The metal is, for example, Ni or a Ni alloy containing Ni as a main component. The metal oxide is, for example, the above-mentioned metal oxide, and may be the same as or different from the metal oxide of the surface coating layer 15.

An anode electrode 10 c shown in FIG. 1C has the same structure as the anode electrode 10 a of FIG. 1A, except that a surface coating layer 15 composed of the above-mentioned metal oxide is disposed on the Al_(x)Ga_(1-x)N layer 11 so as to partially cover the layer 11 (to partially expose the layer 11). The surface coating layer 15 of the anode electrode 10 c is the same as the surface coating layer 15 of the anode electrode 10 b of FIG. 1B, except that the surface of the Al_(x)Ga_(1-x)N layer 11 is covered in a different manner. In the anode electrode 10 c, a plurality of surface coating layers 15 of various shapes and sizes may be arranged regularly or randomly, or a plurality of surface coating layers 15 of the same shape and size may be arranged regularly or randomly.

An anode electrode 10 d shown in FIG. 1D has the same structure as the anode electrode 10 a of FIG. 1A, except that fine particles 16 composed of the above-mentioned metal oxide are arranged on the Al_(x)Ga_(1-x)N layer 11. In the anode electrode 10 d, the function of the metal oxide as a co-catalyst is further enhanced. For more detailed explanation of the metal oxide that is disposed on the Al_(x)Ga_(1-x)N layer 11 of the anode electrode and is in the form of fine particles, see U.S. patent application Ser. No. 13/453,669 filed by the present inventors. The specification of this US patent application is incorporated herein by reference.

In the anode electrode 10 d, a plurality of fine particles 16 of various shapes and sizes may be arranged regularly or randomly, or a plurality of fine particles 16 of the same shape and size may be arranged regularly or randomly.

Both the surface coating layer(s) 15 and the fine particles 16 may be disposed on the Al_(x)Ga_(1-x)N layer 11.

The method for forming the anode electrode is not limited. The Al_(x)Ga_(1-x)N layer 11, the GaN layer 12, and the electrode layer 14 can each be formed on the substrate 13 by a known thin film forming technique. The thin film forming technique is not limited to a specific one. In order to form the GaN layer 12 on the substrate 13 and to form the Al_(x)Ga_(1-x)N layer 11 on the GaN layer 12 thus formed, for example, metal-organic vapor-phase epitaxy, molecular-beam epitaxy, or sputtering can be used. In order to form the electrode layer 14, for example, vacuum deposition, electron beam evaporation, or sputtering can be used.

In order to dispose the metal oxide (the surface coating layer(s) 15 and/or the fine particles 16) on the Al_(x)Ga_(1-x)N layer 11, for example, a method (metal particle coating) in which a solution containing metal oxide particles, such as a slurry solution in which the particles are dispersed, is applied, heated and dried, or a method (metal organic compound decomposition) in which a solution of an organic metal compound is applied by spin coating or the like, and the compound is heated and decomposed, can be used.

In the case where the anode electrode for use in the device and the method of the present disclosure includes a substrate, the substrate may be the electrically conductive substrate shown in each of FIG. 1A to FIG. 1D or an insulating substrate composed of an insulating material. The insulating substrate serves as a layer for increasing the strength, shape retention, and ease of handling of the anode electrode. FIG. 2A to FIG. 2D each show an example of the anode electrode including an insulating substrate.

Anode electrodes 20 a, 20 b, 20 c, and 20 d shown in FIG. 2A, FIG. 2B, FIG. 2C, and FIG. 2D have the same structures as the anode electrodes 10 a, 10 b, 10 c, and 10 d shown in FIG. 1A, FIG. 1B, FIG. 1C, and FIG. 1D, respectively, except that an insulating substrate 23 is used instead of the electrically conductive substrate 13 and that the electrode layer 14 serving as a terminal for extracting electrons generated in the Al_(x)Ga_(1-x)N layer 11 from the anode electrode is disposed on the GaN layer 12 instead of on the substrate 23 due to its insulating property.

The insulating substrate 23 is composed of, for example, sapphire (typically, single-crystal sapphire) or high-resistance silicon.

The method for forming the anode electrodes 20 a to 20 d each including the insulating substrate 23 is the same as the method for forming the anode electrodes 10 a to 10 d each including the electrically conductive substrate 13.

The shape of the anode electrode is not limited, but, it may be plate-shaped.

The anode electrode may optionally include a member other than the above-described members as long as it has the above-described photochemical reaction region where carriers (electrons and holes) are generated by light irradiation, the generated electrons can be extracted from the anode electrode in order to supply them to the cathode electrode, and the generated holes and water react with each other in the photochemical reaction region to produce oxygen and protons.

(Cathode Electrode)

The cathode electrode is composed of a metal or a metal compound. The cathode electrode has a structure capable of receiving electrons generated by photoexcitation in the anode electrode. The cathode electrode has a CO₂ reduction reaction region where CO₂ contained in the first electrolyte solution is reduced by reaction between the electrons received and protons contained in the first electrolyte solution. The cathode electrode has a structure capable of supplying the received electrons to the reduction reaction region. The structure of the cathode electrode is not limited as long as these conditions are satisfied. For example, the cathode electrode may include a portion composed of an insulating material.

The metal that can constitute the cathode electrode, in particular, the metal that can constitute the CO₂ reduction reaction region is, for example, at least one selected from Cu, Au, Ag, tantalum (Ta), and indium (In). The metal may be an alloy. The metal compound that can constitute the cathode electrode, in particular, the metal compound that can constitute the CO₂ reduction reaction region is, for example, at least one selected from tantalum carbide and tantalum nitride. The cathode electrode may include any of these metals or metal compounds, as the whole or a part of the surface of the cathode electrode, only in the CO₂ reduction reaction region. In this case, the other part of the cathode electrode is composed of an arbitrary electrically conductive material and/or an arbitrary insulating material. The other part is, for example, the substrate of the cathode electrode. The substrate is composed of, for example, glass or glassy carbon. Glassy carbon has electrical conductivity. The cathode electrode may have a structure in which particles or fine particles of the metal or the metal compound are dispersedly arranged on the surface of the substrate. In this case, these particles or fine particles serve as the CO₂ reduction reaction region.

In addition, when the reduction reaction region of the cathode electrode is composed of any of these metals or the metal compounds, it is possible to obtain, as a CO₂ reduction product, at least one selected from hydrocarbons such as methane and ethylene, alcohols such as methanol and ethanol, organic acids such as formic acid, aldehydes such as acetaldehyde, and carbon monoxide. It is also possible to selectively produce a CO₂ reduction product by selecting the type of the metal or the metal compound. For example, when the reduction reaction region is composed of Cu, hydrocarbon and/or alcohol is obtained as a CO₂ reduction product. When the reduction reaction region is composed of In, formic acid is obtained selectively as a CO₂ reduction product. Presumably, this is because the state in which CO₂ molecules are adsorbed to the reduction reaction region varies depending on the type of the metal constituting the region.

The shape of the cathode electrode is not limited. For example, the cathode electrode is plate-shaped. From the viewpoint of increasing the effective reaction area, it is desirable that the cathode electrode have the shape of a plate having fine irregularities on its surface or a porous plate.

The method for forming the cathode electrode is not limited, and any known method can be used. In the case where the above-mentioned metal or metal compound is used only for the CO₂ reduction reaction region, for example, the region is formed on the whole or a part of the surface of the substrate, and in this case, any known thin film formation technique and fine particle formation technique can be used to form the region.

(CO₂ Reduction Device)

FIG. 3 shows an example of a CO₂ reduction device of the present disclosure. A device 300 of FIG. 3 includes a cathode chamber 302, an anode chamber 305, and a proton conducting membrane 306. The cathode chamber 302 and the anode chamber 305 are connected to each other in a connecting portion 313. The proton conducting membrane 306 is disposed in the connecting portion 313 between the cathode chamber 302 and the anode chamber 305.

A first electrolyte solution 307 that contains CO₂ is held in the cathode chamber 302. A second electrolyte solution 308 is held in the anode chamber 305. A cathode electrode 301 is disposed in the cathode chamber 302 so as to be in contact with the first electrolyte solution 307. An anode electrode 304 is disposed in the anode chamber 305 so as to be in contact with the second electrolyte solution 308.

The cathode electrode 301 is the cathode electrode for use in the device and method of the present disclosure, as described above, having a CO₂ reduction reaction region therein. The anode electrode 304 is the anode electrode for use in the device and method of the present disclosure, as described above, having a photochemical reaction region therein. The cathode electrode 301 is disposed in the cathode chamber 302 so that at least a part of the CO₂ reduction reaction region (desirably the entire CO₂ reduction reaction region) in the cathode electrode 301 be in contact with the first electrolyte solution 307. The anode electrode 304 is disposed in the anode chamber 305 so that at least a part of the photochemical reaction region (desirably the entire photochemical reaction region) in the anode electrode 304 be in contact with the second electrolyte solution 308. In the example shown in FIG. 3, both the electrodes 301 and 304 are partially immersed in the electrolyte solutions 307 and 308 respectively. The electrodes 301 and/or the electrode 304 may be entirely immersed in the electrolyte solution(s).

In addition, the anode electrode 304 is disposed in the anode chamber 305 in such a manner that the Al_(x)Ga_(1-x)N layer in the photochemical reaction region can be exposed to light. In the example shown in FIG. 3, the anode chamber 305 is provided with a window (not shown) at a part of the chamber, and the photochemical reaction region of the anode electrode 304 is irradiated with light emitted from a light source 303 and passing through the window.

The cathode electrode 301 and the anode electrode 304 are electrically connected to each other by electrode terminals 310 and 311 and a wire 312 connecting the terminals 310 and 311. There is no external power source such as a solar cell or a potentiostat connected between the cathode electrode 301 and the anode electrode 304. This means that the cathode electrode 301 and the anode electrode 304 are electrically connected to each other without any external power source interposed therebetween. The wire 312 serves as a path of electrons generated by photoexcitation in the photochemical reaction region of the anode electrode 304.

The proton conducting membrane 306 serves as a separator between the first electrolyte solution 307 and the second electrolyte solution 308 and separates the first and second electrolyte solutions 307 and 308 from each other. This means that in the device 300, the first electrolyte solution 307 in the cathode chamber 302 and the second electrolyte solution 308 in the anode chamber 305 are not mixed together as long as the proton conducting membrane 306 normally functions. The electrolyte solutions 307 and 308 and the proton conducting membrane 306 serve as a proton diffusion path.

In the device 300, upon irradiation of the photochemical reaction region of the anode electrode 304 as a photochemical electrode with light, carriers (electrons and holes) are generated and oxygen is generated. As described above, based on a multilayer structure of a GaN layer and an Al_(x)Ga_(1-x)N layer containing a specific amount of Mg in the photochemical reaction region, it is possible to achieve a high level of photoelectromotive force and a high carrier utilization efficiency in the anode electrode 304. Thereby, in the device 300, CO₂ can be reduced to carbon compounds with high efficiency without the use of any external power source. It should be understood that these carbon compounds do not contain CO₂ itself. Two or more carbon compounds can be produced by CO₂ reduction.

Electrons generated in the anode electrode 304 move to the reduction reaction region of the cathode electrode 301 and react with CO₂ to reduce CO₂ in that region. As described above, the structure of this region composed of a metal or a metal compound also contributes to the high CO₂ reduction efficiency in the device 300.

The shape of the cathode electrode 301 and that of the anode electrode 304 are not particularly limited. The device 300 may two or more cathode electrodes 301 and/or two or more anode electrodes 304.

The material constituting the cathode chamber 302 and that constituting the anode chamber 305 are not limited as long as they are not significantly corroded by the electrolyte solutions held in these chambers. The material is, for example, a metal like stainless steel, a glass, a resin, or a composite material of these. However, for the material of the anode chamber 305, light irradiation of the photochemical reaction region of the anode electrode 304 has to be considered. However, in the case where the light source 303 is disposed inside the anode chamber 305, such a consideration is not always necessary.

The internal shape of the cathode chamber 302 and that of the anode chamber 305 are not particularly limited.

The cathode chamber 302 and/or the anode chamber 305 may be structured to have a sealable interior space. The sealing of the interior space of the chamber is achieved by a valve, for example.

The first electrolyte solution 307 is not limited as long as it can contain CO₂, is proton-conductive, does not significantly inhibit (desirably does not inhibit) CO₂ reduction reaction at the cathode electrode 301, and does not significantly corrode (desirably does not corrode) the cathode electrode 301. The first electrolyte solution 307 is typically an aqueous solution. The first electrolyte solution 307 is, for example, an aqueous solution containing at least one electrolyte selected from potassium bicarbonate, sodium bicarbonate, potassium chloride, and sodium chloride.

The types of the carbon compounds generated by CO₂ reduction and the ratio of the compounds generated may vary depending on the type of the electrolyte.

The concentration of the electrolyte in the first electrolyte solution 307 is desirably 1 mol/L or more, and more desirably 3 mol/L or more. The upper limit of the concentration is not particularly limited, and it is 5 mol/L, for example.

The first electrolyte solution 307 contains CO₂. The concentration of CO₂ contained therein is not limited. Desirably, the first electrolyte solution 307 containing CO₂ dissolved therein is acidic.

The device 300 can be operated using the first electrolyte solution 307 previously containing CO₂. The device 300 can be operated while a gas containing CO₂ is supplied into the first electrolyte solution 307. In the example shown in FIG. 3, the device 300 is operated while a gas containing CO₂ is supplied into the first electrolyte solution 307 through a gas supply pipe 309. The gas containing CO₂ may be pure CO₂ (100% CO₂ gas).

The second electrolyte solution 308 is not limited as long as it is proton-conductive, does not significantly inhibit (desirably does not inhibit) the photochemical reaction at the anode electrode 304, and does not significantly corrode (desirably does not corrode) the anode electrode 304. The second electrolyte solution 308 is typically an aqueous solution. The second electrolyte solution 308 is, for example, an aqueous sodium hydroxide solution.

The concentration of the electrolyte in the second electrolyte solution 308 is desirably 1 mol/L or more, and more desirably 5 mol/L or more. The upper limit of the concentration is not particularly limited, and it is 8 mol/L, for example. Desirably, the second electrolytic solution 308 is strongly basic.

The material constituting the proton conducting membrane 306 is not limited as long as the membrane 306 is permeable to protons and can serve as a separator between the first and second electrolyte solutions. Desirably, the proton conducting membrane 306 is a membrane impermeable to the electrolytes contained in the electrolyte solutions 307 and 308. The material is, for example, a proton conducting polymer material, and a specific example thereof is perfluorocarbon sulfonic acid like Nafion (registered trademark).

It is only necessary that the proton conducting membrane 306 be thick enough to ensure the strength required to serve as a separator between the first and second electrolyte solutions, and the thickness of the membrane 306 is 50 to 200 μm, for example.

The light source 303 emits light having energy required to allow generation of carriers by photoexcitation to proceed in the photochemical reaction region of the anode electrode 304. More specifically, the light source 303 emits light of a wavelength of 365 nm or less (light having a wavelength of 365 nm or less). The light source 303 may emit continuous light containing light component of wavelengths of 365 nm or less. Instead, the light source 303 may be a light source, e.g., a laser, that emits monochromatic light containing light components within a wavelength range of 365 nm or less. Desirably, the light source 303 emits light of a wavelength of 250 nm or more and 325 nm or less.

The light source 303 is, for example, a xenon lamp, a deuterium lamp, a mercury lamp, or a metal halide lamp. Sunlight also can be used as the light source 303.

The method for irradiating the photochemical reaction region of the anode electrode 304 with light from the light source 303 is not limited. In the case where the light source 303 is placed outside the anode chamber 305, the anode chamber 305 needs to have a window for introducing the light emitted from the light source 303 into the anode chamber 305. The light source 303 may be placed inside the anode chamber 305.

The use of the device of the present disclosure is not limited. The device of the present disclosure can be applied to any use that requires or desires CO₂ reduction. Specific examples of the use include formation of carbon compounds from CO₂ as a carbon source, such as carbon monoxide and/or organic compounds like alcohol, aldehyde, carboxylic acid, hydrocarbon, and formation of oxygen.

From another aspect, specific examples of the use include removal of CO₂ from a closed space and supply of oxygen into the space. The device of the present disclosure can also be applied to reduction of CO₂ in the atmosphere to suppress global warming (not only direct reduction of CO₂ but also reduction of CO₂ emissions by reducing fossil fuel consumption using CO₂ as a carbon source), production of oxygen (artificial photosynthesis) as an alternative to plant photosynthesis, etc.

[CO₂ Reduction Method]

The method of the present disclosure is a method for reducing CO₂ using the photochemical electrode described above as an anode electrode. For example, in the method of the present disclosure, CO₂ is reduced by the device of the present disclosure described above. Thereby, CO₂ can be reduced more efficiently than before not using any external power source but using light energy.

The method of the present disclosure can be performed by the CO₂ reduction device 300 shown in FIG. 3. An example of the method of the present disclosure is described with reference to FIG. 3.

As shown in FIG. 3, the Al_(x)Ga_(1-x)N layer 11 in the photochemical reaction region of the anode electrode 304 is irradiated with light having a wavelength of 365 nm or less, with the first electrolyte solution 307 and the second electrolyte solution 308 being held in the cathode chamber 302 and the anode chamber 305 respectively, so as to allow generation of electrons and protons to proceed in that region. Protons are generated by the reaction between water and holes generated in the Al_(x)Ga_(1-x)N layer 11 in the photochemical reaction region. With this reaction, a reaction of reducing CO₂ contained in the first electrolyte solution 307 by electrons generated in the photochemical reaction region of the anode electrode 304 and protons contained in the first electrolyte solution 307 is allowed to proceed in the reduction reaction region of the cathode electrode 301. The generation of electrons and protons at the anode electrode 304 and the reduction of CO₂ at the cathode electrode 301 can proceed simultaneously.

Desirably, the irradiated light has a wavelength of 250 nm or more and 325 nm or less.

The method of the present disclosure may further include the step of introducing a gas containing CO₂ into the first electrolyte solution 307 held in the cathode chamber 302. The method for supplying the gas containing CO₂ into the first electrolyte solution 307 is not limited. In the example shown in FIG. 3, the gas containing CO₂ is supplied into the first electrolyte solution 307 through the gas supply pipe 309 having one end immersed in the first electrolyte solution 307. This step may be performed during the operation of the device 300. That is, it is possible to reduce CO₂ while supplying the gas containing CO₂ into the first electrolyte solution 307. This step may also be performed before the device 300 is operated. Desirably, the gas containing CO₂ is supplied into the first electrolyte solution 307 before the device 300 is operated so as to start the operation of the device 300 after the first electrolyte solution 307 containing a sufficient amount of CO₂ is prepared.

In the method of the present disclosure, at least one selected from alcohols such as methanol and ethanol, aldehydes such as acetaldehyde, organic acids such as formic acid, hydrocarbons such as methane and ethylene, and carbon monoxide is obtained by the above-described reaction of reducing CO₂. The carbon compound generated by the CO₂ reduction can be selected by, for example, the structure of the cathode electrode 301, the type of the first electrolyte solution 307, etc.

In the method of the present disclosure, the CO₂ reduction can be performed with the device 300 being placed at room temperature and atmospheric pressure. This means that the method of the present disclosure does not necessarily require a special environment (such as a high-temperature and high-pressure environment).

Any optional step may be performed in addition to the steps described above as long as the effects of the present invention can be obtained.

The application of the method of the present disclosure is not limited. Specific examples of the application are as described above as specific examples of the use of the device of the present disclosure.

EXAMPLES

Hereinafter, the CO₂ reduction device and the CO₂ reduction method of the present disclosure are described in more detail with reference to Examples. The device and the method of the present disclosure are not limited by the following Examples.

Example 1

In Example 1, a multilayer body of an electrode layer, an electrically conductive substrate, a GaN layer, and a Mg-containing Al_(x)Ga_(1-x)N layer was used as an anode electrode. On the Al_(x)Ga_(1-x)N layer of this multilayer body (on the surface of the Al_(x)Ga_(1-x)N layer opposite to the surface thereof facing the GaN layer), fine particles of Ni-containing metal oxide were dispersedly arranged, as shown in FIG. 1D. The electrically conductive substrate was a single-crystal GaN substrate doped with high-concentration Si (with a thickness of about 0.4 mm). The GaN layer was a Si-doped n*-type GaN layer (with a thickness of 3.0 μm and a Si doping concentration of 4.0×10¹⁸ in terms of the number of atoms per cm³). The thickness of the Al_(x)Ga_(1-x)N layer was 100 nm, the value of x was 0.10, and the Mg doping concentration was 1.0×10¹⁷ in terms of the number of atoms per cm³. The metal oxide fine particles were fine particles of nickel oxide (with a diameter of several tens of nanometers to several micrometers), and they were dispersedly arranged on the Al_(x)Ga_(1-x)N layer in such a manner that the Al_(x)Ga_(1-x)N layer is partially exposed. A solution containing a Ni compound dispersed therein was applied onto the surface of the Al_(x)Ga_(1-x)N layer and then fired, and thereby the metal oxide fine particles were arranged on the surface of this layer. The number of fine particles placed thereon was about 1×10⁸ to 1×10¹⁰ per unit area of 1 cm².

The GaN layer was formed on the single-crystal GaN substrate by growing GaN thereon by metal-organic vapor-phase epitaxy. The Mg-containing Al_(x)Ga_(1-x)N layer was formed on the GaN layer thus formed by growing Mg-containing Al_(x)Ga_(1-x)N thereon by metal-organic vapor-phase epitaxy.

The electrode layer was a multilayer body of Ti/Al/Au (with a thickness of 500 nm). A multilayer body of the electrically conductive substrate, the GaN layer, and the Mg-containing Al_(x)Ga_(1-x)N layer was formed, the nickel oxide fine particles were arranged on the Al_(x)Ga_(1-x)N layer of the multilayer body, and then the electrode layer was formed on the surface of the electrically conductive substrate opposite to the surface thereof facing the GaN layer by electron beam evaporation. The electrode layer was formed in such a manner that the Ti film was in contact with the electrically conductive substrate to increase the adhesion between the electrode layer and the single-crystal GaN substrate and to reduce the interfacial resistance therebetween.

On the other hand, a copper plate (with a thickness of 0.5 mm) was used as a cathode electrode.

A CO₂ reduction device shown in FIG. 3 was fabricated using the anode electrode and the cathode electrode prepared as described above. More specific configuration and operation conditions of the CO₂ reduction device thus fabricated were as follows.

Cathode Chamber

Cathode electrode: Copper plate (with a thickness of 0.5 mm)

First electrolyte solution: 180 cm³ of aqueous potassium bicarbonate solution having a concentration of 3.0 mol/L

Area of the cathode electrode immersed in the first electrolyte solution: about 4 cm²

Supply of CO₂: CO₂ was supplied into the first electrolyte solution at a flow rate of 200 mL/minute for 30 minutes through the gas supply pipe 309 shown in FIG. 3 before the anode electrode was irradiated with light. After CO₂ was supplied, the cathode chamber was sealed to prevent leakage of CO₂ outside the cathode chamber.

Anode Chamber

Anode electrode: Multilayer body prepared as above

Second electrolyte solution: 180 cm³ of aqueous sodium hydroxide solution having a concentration of 5.0 mol/L

Light irradiation: A quartz glass window (not shown in FIG. 3) was provided in the anode chamber so as to irradiate the Al_(x)Ga_(1-x)N layer of the anode electrode with light from outside the chamber.

Connection Between Anode Chamber and Cathode Chamber

The anode chamber and the cathode chamber were connected at a distance of about 8 cm from each other. The area of the connecting portion was about 12.5 cm³, and as a proton conducting membrane serving as a separator between the first electrolyte solution and the second electrolyte solution, a Nafion membrane (“Nafion 117” with a thickness of about 180 μm, manufactured by DuPont) was disposed in the connecting portion.

Connection Between Anode Electrode and Cathode Electrode

The electrode layer of the anode electrode and the end of the copper plate as the cathode electrode were electrically connected by the wire 312 without any external power source like a battery or a potentiostat disposed between these electrodes. An ammeter was disposed between the anode electrode and the cathode electrode to detect a current flowing between these electrodes upon irradiation with light.

Light Source

As a light source, a xenon lamp (with a power of 300 W, a light irradiation area of about 4 cm², and an irradiation light power of about 20 mW/cm²) was used. Light emitted from this light source has a broad spectrum within a wavelength range of 365 nm or less.

In this reduction device, CO₂ gas was supplied into the cathode chamber and then the Al_(x)Ga_(1-x)N layer of the anode electrode was irradiated with light. As a result, a current flowing from the cathode electrode to the anode electrode was detected, that is, the flow of electrons from the anode electrode to the cathode electrode was observed, and the evolution of a gas from the surface of the Al_(x)Ga_(1-x)N layer of the anode electrode was observed. When the light irradiation was temporarily stopped, the current was not detected and the evolution of the gas also stopped. When the irradiation was resumed, the current was detected again and the gas was evolved again. It was thus confirmed that upon irradiation of the Al_(x)Ga_(1-x)N layer of the anode electrode with light, some kind of chemical reaction proceeds at the anode electrode and the cathode electrode.

Next, the gas evolved at the anode electrode was found to be oxygen when examined separately. After the light irradiation, in order to identify carbon compounds contained in the first electrolyte solution, the gas components of the compounds were analyzed using a gas chromatograph (GC) (GC-4000 manufactured by GL Sciences Inc.) and the liquid components of the compounds were analyzed using a liquid chromatograph (LC) (LC-2010 manufactured by Shimadzu Corporation) and GC with head-space sampler (HS-GC) (GC-17A manufactured by Shimadzu Corporation and HS40 manufactured by Perkin-Elmer Corporation). As a result, carbon monoxide and formic acid were detected. It was thus confirmed that upon irradiation of the Al_(x)Ga_(1-x)N layer of the anode electrode with light, CO₂ contained in the first electrolyte solution in the cathode chamber was reduced and carbon monoxide and formic acid were produced. The amounts of carbon monoxide and formic acid produced were determined by GC and LC. These amounts increased in proportion to the time of light irradiation of the anode electrode.

Comparative Example 1

A CO₂ reduction device was fabricated in the same manner as in Example 1, except that an Al_(x)Ga_(1-x)N layer not containing Mg was used for the anode electrode, and the device thus fabricated was irradiated with light in the same manner as in Example 1.

In Comparative Example 1, as in the case of Example 1, it was confirmed that upon irradiation of the Al_(x)Ga_(1-x)N layer of the anode electrode with light, a gas was evolved from the surface of the Al_(x)Ga_(1-x)N layer of the anode electrode and that carbon monoxide and formic acid were produced by the reduction of CO₂ contained in the first electrolyte solution in the cathode chamber.

Next, the value of current flowing between the electrodes during light irradiation in Example 1 was compared with that in Comparative Example 1. As a result, the value of current in Example 1 was about twice the value of current in Comparative Example 1. The amount of CO₂ reduction products (carbon monoxide and formic acid) produced in a given period of irradiation time in Example 1 was compared with the amount of CO₂ reduction products produced in the same period of irradiation time in Comparative Example 1. As a result, the production amount in Example 1 was about twice the production amount in Comparative Example 1. That is, the value of the reaction current and the amount of the reaction products obtained when an anode electrode having an Al_(x)Ga_(1-x)N layer containing Mg was used were about twice those obtained when an anode electrode having an Al_(x)Ga_(1-x)N layer not containing Mg was used, and thus it was confirmed that CO₂ can be reduced more efficiently by using the former anode electrode.

Example 2

A CO₂ reduction device was fabricated in the same manner as in Example 1, except that fine particles of nickel oxide were not arranged on the Al_(x)Ga_(1-x)N layer of the anode electrode, and the device thus fabricated was irradiated with light in the same manner as in Example 1.

In Example 2, as in the case of Example 1, it was confirmed that upon irradiation of the Al_(x)Ga_(1-x)N layer of the anode electrode with light, a gas was evolved from the surface of the Al_(x)Ga_(1-x)N layer of the anode electrode and that carbon monoxide and formic acid were produced by the reduction of CO₂ contained in the first electrolyte solution in the cathode chamber.

Next, the value of current flowing between the electrodes during light irradiation in Example 2 was compared with that in Example 1. The value of current in Example 2 was almost equal to that in Example 1. The amount of CO₂ reduction products (carbon monoxide and formic acid) produced in a given period of irradiation time in Example 2 was compared with the amount of CO₂ reduction products produced in the same period of irradiation time in Example 1. As shown in FIG. 4, the amount of reduction products obtained in Example 2 was slightly smaller than that in Example 1 but was sufficiently greater than that in Comparative Example 1. FIG. 4 shows the amounts of CO₂ reduced (=the amount of CO₂ reduction products produced) per unit time in Examples 1 to 3, relative to the amount of CO₂ reduced per unit time in Comparative Example 1.

Example 3

In Example 3, a CO₂ reduction device was fabricated in the same manner as in Example 1, except that an insulating substrate (single-crystal sapphire substrate with a thickness of about 0.4 mm) was used instead of an electrically conductive substrate as the substrate of the anode electrode and that the electrode layer was placed on the GaN layer (see FIG. 2D) instead of on the surface of the substrate opposite to the surface thereof facing the GaN layer. The electrode layer was placed in such a manner that the Ti film was in contact with the GaN layer to increase the adhesion between the electrode layer and the GaN layer and to reduce the interfacial resistance therebetween.

The Al_(x)Ga_(1-x)N layer of the anode electrode in the CO₂ reduction device thus fabricated was irradiated with light in the same manner as in Example 1. As a result, it was confirmed that a gas was evolved from the surface of the Al_(x)Ga_(1-x)N layer of the anode electrode and that carbon monoxide and formic acid were produced by the reduction of CO₂ contained in the first electrolyte solution in the cathode chamber.

Next, the value of current flowing between the electrodes during light irradiation in Example 3 was compared with that in Example 1. The value of current in Example 3 was almost equal to that in Example 1. The amount of CO₂ reduction products (carbon monoxide and formic acid) produced in a given period of irradiation time in Example 3 was compared with the amount of CO₂ reduction products produced in the same period of irradiation time in Example 1. As shown in FIG. 4, the amount of reduction products obtained in Example 3 was slightly smaller than that in Example 1 but was sufficiently greater than that in Comparative Example 1 and slightly greater than that in Example 2.

As shown in FIG. 4, it was confirmed that more efficient CO₂ reduction can be achieved by the use of the Mg-containing Al_(x)Ga_(1-x)N layer in the anode electrode.

Example 4

A CO₂ reduction device was fabricated in the same manner as in Example 1, except that as a cathode electrode, an electrode obtained by dispersing copper fine particles (with a diameter of 20 nm to 100 nm) on the surface of a glassy carbon substrate (Glassy Carbon (registered trademark) with a thickness of 0.5 mm, manufactured by Tokai Carbon, Co., Ltd.) in such a manner that part of the surface of the substrate was exposed was used instead of a copper plate, and the device thus fabricated was irradiated with light in the same manner as in Example 1. A dispersion of a copper compound was applied onto the surface of the substrate by spin coating, dried to remove organic components, and then fired in a reducing atmosphere. Thereby, the copper fine particles were placed on the surface of the substrate. The number of the fine particles placed thereon was about 1×10⁸ to 4×10⁹ per unit area of 1 cm².

In Example 4, as in the case of Example 1, it was confirmed that upon irradiation of the Al_(x)Ga_(1-x)N layer of the anode electrode with light, a gas was evolved from the surface of the Al_(x)Ga_(1-x)N layer of the anode electrode and that carbon monoxide and formic acid were produced by the reduction of CO₂ contained in the first electrolyte solution in the cathode chamber. The value of current flowing between the electrodes during light irradiation in Example 4 was compared with that in Example 1. The value of current in Example 4 was almost equal to that in Example 1.

When a cathode electrode in which instead of copper fine particles, fine particles of a copper-nickel alloy containing traces of nickel component were placed was used, almost the same results as the case where copper fine particles were placed was obtained.

Example 5

A CO₂ reduction device was fabricated in the same manner as in Example 1, except that an indium plate (with a thickness of 0.5 mm) was used instead of a copper plate as a cathode electrode, and the device thus fabricated was irradiated with light in the same manner as in Example 1.

In Example 5, as in the case of Example 1, it was confirmed that a gas was evolved from the surface of the Al_(x)Ga_(1-x)N layer of the anode electrode upon irradiation of the Al_(x)Ga_(1-x)N layer of the anode electrode. The value of current flowing between the electrodes during light irradiation in Example 5 was compared with that in Example 1. As a result, the value of current in Example 5 was almost equal to that in Example 1. On the other hand, after the light irradiation, the carbon compounds contained in the first electrolyte solution were analyzed by GC and LC, and as a result, it was confirmed that most of the compounds was formic acid. This means that formic acid was selectively produced as a CO₂ reduction product by using indium as a cathode electrode.

Example 6

A CO₂ reduction device was fabricated in the same manner as in Example 1, except that an aqueous potassium chloride solution (having a concentration of 3.0 mol/L) was used instead of an aqueous potassium bicarbonate solution as a first electrolyte solution, and the device thus fabricated was irradiated with light in the same manner as in Example 1.

In Example 6, as in the case of Example 1, it was confirmed that upon irradiation of the Al_(x)Ga_(1-x)N layer of the anode electrode with light, a gas was evolved from the surface of the Al_(x)Ga_(1-x)N layer of the anode electrode. The value of current flowing between the electrodes during light irradiation in Example 6 was compared with that in Example 1. The value of current in Example 6 was almost equal to that in Example 1. On the other hand, after the light irradiation, the carbon compounds contained in the first electrolyte solution were analyzed by GC and LC, and as a result, it was confirmed that ethylene, alcohols such as ethanol, and acetaldehyde were produced in addition to carbon monoxide and formic acid observed in Example 1.

Almost the same results were obtained when an aqueous sodium chloride solution was used as the first electrolyte solution.

Example 7

In Example 7, a plurality of anode electrodes including Al_(x)Ga_(1-x)N layers having different contents (different doping concentrations) of Mg atoms were prepared. CO₂ reduction devices were fabricated in the same manner as in Example 1, except that the anode electrodes having different Mg contents were used, and the devices thus fabricated were irradiated with light in the same manner as in Example 1.

In Example 7, as in the case of Example 1, it was confirmed that upon irradiation of the Al_(x)Ga_(1-x)N layer of the anode electrode with light, a gas was evolved from the surface of the Al_(x)Ga_(1-x)N layer of the anode electrode and that carbon monoxide and formic acid were produced by the reduction of CO₂ contained in the first electrolyte solution in the cathode chamber.

On the other hand, when the devices were irradiated with light for a given period of time, the amounts of CO₂ reduction products (carbon monoxide and formic acid), that is, the amounts of CO₂ reduced in that period of time varied depending on the types of the anode electrodes (the Mg contents in the Al_(x)Ga_(1-x)N layers) prepared in Example 7. FIG. 5 shows the relationship between the Mg content in the Al_(x)Ga_(1-x)N layer and the amount of CO₂ reduced per unit time. In FIG. 5, the vertical axis indicates the values relative to the value (=1) of the amount of CO₂ that was reduced when an Al_(x)Ga_(1-x)N layer having a Mg content of 0, that is, an Al_(x)Ga_(1-x)N layer not containing Mg was used in an anode electrode (i.e., the amount of CO₂ reduced in Comparative Example 1).

As shown in FIG. 5, relative to the amount of CO₂ that was reduced when an Al_(x)Ga_(1-x)N layer not containing Mg was used, the amount of CO₂ reduced in Example 7 began to increase rapidly when the Mg content in the Al_(x)Ga_(1-x)N layer reached and exceeded 1×10¹⁵ in terms of the number of atoms per cm³. The value of the current flowing between the electrodes also began to increase rapidly with the rapid change in the amount of reduced CO₂. The amount of reduced CO₂ and the current value peaked when the Mg content in the Al_(x)Ga_(1-x)N layer was 1×10¹⁷ in terms of the number of atoms per cm³, and the amount of reduced CO₂ and the current value decreased as the Mg content further increased. When the Mg content in the Al_(x)Ga_(1-x)N layer exceeded 1×10¹⁹ in terms of the number of atoms per cm³, the amount of reduced CO₂ and the current value decreased rapidly. This is presumably because an excessive amount of Mg contained in the Al_(x)Ga_(1-x)N layer had changed the characteristics of the Al_(x)Ga_(1-x)N layer, which affected the utilization efficiency of carriers generated by photoexcitation. As shown in FIG. 5, from the viewpoint of the amount of reduced CO₂, the Mg content in the Al_(x)Ga_(1-x)N layer was desirably 1×10¹⁶ or more and 1×10¹⁸ or less in terms of the number of atoms per cm³. However, the optimum value of the Mg content (1×10¹⁷ in terms of the number of atoms per cm³ in FIG. 5) may vary depending of the value of x in the composition of the Al_(x)Ga_(1-x)N layer as a base material because the optimum value is influenced by the composition and the characteristics of that Al_(x)Ga_(1-x)N layer.

Example 8

In Example 8, 4 types of anode electrodes including Al_(x)Ga_(1-x)N layers having different compositions (having different x values) were prepared. CO₂ reduction devices were fabricated in the same manner as in Example 1, except that the anode electrodes having different compositions were used, and the devices thus fabricated were irradiated with light in the same manner as in Example 1. The x values were 0.05, 0.10, 0.15, and 0.20, respectively.

In Example 8, as in the case of Example 1, it was confirmed that upon irradiation of the Al_(x)Ga_(1-x)N layer of the anode electrode with light, a gas was evolved from the surface of the Al_(x)Ga_(1-x)N layer of the anode electrode and that carbon monoxide and formic acid were produced by the reduction of CO₂ contained in the first electrolyte solution in the cathode chamber. When the devices of Example 8 were irradiated with light for a given period of time, the amounts of CO₂ reduction products produced in these devices including the above-mentioned anode electrodes were almost the same as the amount of CO₂ reduction products produced in Example 1.

As shown in Examples 1 to 8 and Comparative Example 1, it was confirmed that the use of an anode electrode including a photochemical reaction region composed of nitride semiconductors, more specifically, having a multilayer structure of a GaN layer and an Al_(x)Ga_(1-x)N layer containing (doped with) a specific amount of Mg atoms, makes it possible to increase the value of reaction current obtained by irradiation of the anode electrode with light and to reduce CO₂ with high efficiency at a cathode electrode.

The present disclosure may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this specification are to be considered in all respects as illustrative and not limiting. The scope of the present disclosure is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

INDUSTRIAL APPLICABILITY

The device of the present disclosure can be applied to all industries that require CO₂ reduction or desire CO₂ reduction. These industries include the space industry, more specifically removal of CO₂ from a spacecraft or a space station in the extraterrestrial space. This device can also be applied to the production of a wide variety of substances that can be obtained by CO₂ reduction, such as alcohol, aldehyde, carboxylic acid, hydrocarbon, carbon monoxide, and oxygen. This device can further be applied to reduction of CO₂ in the atmosphere to suppress global warming, production of oxygen as an alternative to plant photosynthesis, etc. 

What is claimed is:
 1. A carbon dioxide reduction device for reducing carbon dioxide by light energy, comprising: a cathode chamber holding a first electrolyte solution that contains carbon dioxide; an anode chamber holding a second electrolyte solution, connected to the cathode chamber; a proton conducting membrane that is disposed in a connecting portion between the anode chamber and the cathode chamber so as to serve as a separator between the first electrolyte solution and the second electrolyte solution and to conduct hydrogen ions between the first and second electrolyte solutions; a cathode electrode disposed in the cathode chamber so as to be in contact with the first electrolyte solution; and an anode electrode disposed in the anode chamber so as to be in contact with the second electrolyte solution, wherein the cathode electrode has a carbon dioxide reduction reaction region that is in contact with the first electrolyte solution and is composed of a metal or a metal compound, the anode electrode has a photochemical reaction region that is in contact with the second electrolyte solution and is composed of nitride semiconductors, the photochemical reaction region of the anode electrode has a multilayer structure of a GaN layer and an Al_(x)Ga_(1-x)N layer containing Mg (0<x≦0.25), a content of Mg in the Al_(x)Ga_(1-x)N layer is 1×10¹⁵ or more and 1×10¹⁹ or less in terms of the number of Mg atoms that are contained in a unit volume (1 cm³) of the Al_(x)Ga_(1-x)N layer, the anode electrode is disposed in the anode chamber in such a manner that the Al_(x)Ga_(1-x)N layer in the photochemical reaction region can be exposed to light, and the cathode electrode and the anode electrode are electrically connected to each other without an external power source interposed therebetween.
 2. The carbon dioxide reduction device according to claim 1, wherein the content of Mg in the Al_(x)Ga_(1-x)N layer is 1×10¹⁶ or more and 1×10¹⁸ or less in terms of the number of Mg atoms that are contained in the unit volume (1 cm³) of the Al_(x)Ga_(1-x)N layer.
 3. The carbon dioxide reduction device according to claim 1, wherein the x has a value of 0.10 or more and 0.15 or less.
 4. The carbon dioxide reduction device according to claim 1, wherein the GaN layer is composed of an n-type GaN.
 5. The carbon dioxide reduction device according to claim 1, wherein a metal oxide containing Ni is disposed on the Al_(x)Ga_(1-x)N layer in the photochemical reaction region.
 6. The carbon dioxide reduction device according to claim 5, wherein the metal oxide is in the form of fine particles.
 7. The carbon dioxide reduction device according to claim 1, wherein the metal constituting the reduction reaction region includes at least one selected from copper, gold, silver, tantalum, and indium.
 8. The carbon dioxide reduction device according to claim 1, wherein the first electrolyte solution is an aqueous solution containing at least one electrolyte selected from potassium bicarbonate, sodium bicarbonate, potassium chloride, and sodium chloride.
 9. A method for reducing carbon dioxide using a carbon dioxide reduction device, wherein the device is the carbon dioxide reduction device according to claim 1, and the method comprises the step of irradiating the Al_(x)Ga_(1-x)N layer in the photochemical reaction region of the anode electrode with light having a wavelength of 365 nm or less, with the first electrolyte solution and the second electrolyte solution being held in the cathode chamber and the anode chamber respectively, so as to allow generation of electrons and hydrogen ions to proceed in the photochemical reaction region and to allow a reaction of reducing carbon dioxide contained in the first electrolyte solution to proceed in the reduction reaction region of the cathode electrode.
 10. The method for reducing carbon dioxide according to claim 9, further comprising the step of introducing a gas containing carbon dioxide into the first electrolyte solution held in the cathode chamber.
 11. The method for reducing carbon dioxide according to claim 9, wherein the step is performed with the device being placed at room temperature and atmospheric pressure.
 12. The method for reducing carbon dioxide according to claim 9, wherein the reaction of reducing carbon dioxide produces at least one selected from methanol, ethanol, acetaldehyde, formic acid, methane, ethylene, and carbon monoxide. 